Yttria-stabilized zirconia (YSZ) is a well known material used to improve the performance of metals used in high temperature metals. The YSZ is applied, typically by a high temperature thermal spray coating process, as a thermal barrier coating (TBC). The TBC increases the operating temperature of the high temperature substrate metal. In addition, a bond coat is applied between the TBC and the high temperature metal to reduce the thermal mismatch between the TBC and the high temperature metal, which improves the spallation resistance of the TBC. This thermal barrier coating system includes the bond coat and the TBC. The bond coat typically is a MCrAlY, where M is a metal selected from the group consisting of Ni, Co, Fe and combinations thereof.
Thermal barrier coating systems are commonly used in the hot section of gas turbine engines to improve the temperature performance of high temperature substrate metals. These high temperature substrate metals are used for components such as combustors and turbine blades. The high temperature metals commonly are superalloy metals, and can be nickel based superalloy, cobalt based superalloys, iron based superalloys and combinations thereof. The TBC systems significantly increase the high temperature performance range of these superalloy metals.
Gas turbine engines can be operated using a number of different fuels. These fuels are combusted in the combustor section of the engine at temperatures at or in excess of 2000° F. (1093° C.), and the gases of combustion are used to rotate the turbine section of the engine, located aft of the combustor section of the engine. Power is generated by the rotating turbine section as energy is extracted from the hot gases of combustion. It is generally economically beneficial to operate the gas turbine engines using the most inexpensive fuel available. One of the more abundant and inexpensive petroleum fuels is heavy fuel oil (HFO). One of the reasons that HFO is an economical fuel is that it is not heavily refined. Not being heavily refined, it contains a number of impurities. One of these impurities is vanadium, which forms vanadium oxide (V2O5) at the high temperatures of combustion. Even though MgO is added as a fuel additive and acts as an inhibitor for reaction of vanadium species that forms an inert magnesium vanadate compound on or near the outer surface of the thermal barrier coating, MgO does not completely prevent the attack of YSZ thermal barrier coatings, as vanadium oxide can penetrate microcracks and porosity in the thermal barrier coatings, providing access not only to the YSZ thermal barrier coating, but also the underlying bond coat. V2O5 is an acidic oxide that can leach yttria from YSZ in cracks and porosity that occur in such thermal barrier coatings. The mechanism of attack is provided by the following reaction:ZrO2(Y2O3)+V2O5→ZrO2 (monoclinic)+2YVO4 Thus, V2O5 maintains the ability to rapidly attack YSZ, causing it to deteriorate and be removed by the hot gas stream. The loss of the TBC exposes the substrate metal and any remaining bond coat to the hot gases of combustion at elevated temperatures. At these elevated temperatures, the substrate metal and the bond coat is subject to corrosion from the hot gases of combustion, which shortens their life. As a result, the components such as combustors and turbine blades must be replaced in shorter intervals, which also means additional maintenance time for the turbine during which time it is not producing power.
A thermal barrier coating system that can be applied to hot gas path components in gas turbine engines that is not subject to attack by the hot gases of combustion is desirable in the art. In particular, the coating system should be resistant to attack by V2O5 that may be generated by HFO, while at the same time providing thermal protection to the substrate metal so that its life expectancy can be improved.